Volume Measurement Using Gas Laws

ABSTRACT

A system and method for measuring fluid volume and changes in fluid volume over time, using a simple, low cost architecture is described.

BACKGROUND

The present disclosure relates to fluid flow control devices and moreparticularly to feedback control infusion pumps.

The primary role of an intravenous (IV) infusion device has beentraditionally viewed as a way of delivering IV fluids at a certain flowrate. In clinical practice, however, it is common to have fluid deliverygoals other than flow rate. For example, it may be important to delivera certain dose over an extended period of time, even if the startingvolume and the actual delivery rate are not specified. This scenario of“dose delivery” is analogous to driving an automobile a certain distancein a fixed period of time by using an odometer and a clock, withoutregard to a speedometer reading. The ability to perform accurate “dosedelivery” would be augmented by an ability to measure the volume ofliquid remaining in the infusion.

Flow control devices of all sorts have an inherent error in theiraccuracy. Over time, the inaccuracy of the flow rate is compounded, sothat the actual fluid volume delivered is further and further from thetargeted volume. If the volume of the liquid to be infused can bemeasured, then this volume error can be used to adjust the deliveryrate, bringing the flow control progressively back to zero error. Theability to measure fluid volume then provides an integrated error signalfor a closed feedback control infusion system.

In clinical practice, the starting volume of an infusion is not knownprecisely. The original contained volume is not a precise amount andthen various concentrations and mixtures of medications are added. Theresult is that the actual volume of an infusion may range, for example,from about 5% below to about 20% above the nominal infusion volume. Thenurse or other user of an infusion control device is left to play a gameof estimating the fluid volume, so that the device stops prior tocompletely emptying the container, otherwise generating an alarm for airin the infusion line or the detection of an occluded line. This processof estimating often involves multiple steps to program the “volume to beinfused.” This process of programming is time consuming and presents anunwanted opportunity for programming error. Therefore, it would bedesirable if the fluid flow control system could measure fluid volumeaccurately and automatically.

If fluid volume can be measured then this information could be viewed asit changes over time, providing information related to fluid flow rates.After all, a flow rate is simply the measurement of volume change overtime.

The formulation of the ideal gas law, PV=nRT, has been commonly used tomeasure gas volumes. One popular method of using the gas law theory isto measure the pressures in two chambers, one of known volume and theother of unknown volume, and then to combine the two volumes and measurethe resultant pressure. This method has two drawbacks. First, thechamber of known volume is a fixed size, so that the change in pressureresultant from the combination of the two chambers may be too small ortoo large for the measurement system in place. In other words, theresolution of this method is limited. Second, the energy efficiency ofthis common measurement system is low, because the potential energy ofpressurized gas in the chambers is lost to atmosphere during thetesting. The present invention contemplates an improved volumemeasurement system and method and apparatus that overcome theaforementioned limitations and others.

SUMMARY

In one aspect, a method for determining the volume of fluid remaining inan infusion is provided.

In another aspect, a method for determining fluid flow rate over anextended period of time is provided.

In another aspect, a method for determining fluid flow rate over arelatively short period of time is provided.

One advantage of the present disclosure is that long term doses can bedelivered on time, because the remaining fluid volume can measured, sothat flow rate errors do not accumulate over time.

Another advantage of the present disclosure is that nurses or otherusers of the infusion system will not have to estimate, enter, andre-enter the volume to be infused. This will reduce the workload for theuser and will eliminate opportunities for programming error.

Another advantage is found in that volume measurements made over timecan be used to accurately compute fluid flow rate.

Another advantage is found in that volume measurements may be made usingan inexpensive and simple pumping mechanism.

Another advantage is found in that volume measurements may be madewithout significant loss of energy.

Another advantage is found in that volume measurements may be made overa wide range of volumes.

Another advantage of the present disclosure is that its simplicity,along with feedback control, makes for a reliable architecture.

Other benefits and advantages of the present disclosure will becomeapparent to those skilled in the art upon a reading and understanding ofthe preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating preferred embodiments and are notto be construed as limiting the invention.

FIGS. 1 and 2 are perspective and side views of an infusion pump inaccordance with an exemplary embodiment.

FIG. 3 is a functional block diagram showing the fluidic connections ofa volume measurement system according to an exemplary embodiment.

FIG. 4 is a functional block diagram showing the control elements of avolume measurement system according to an exemplary embodiment.

FIG. 5 is a functional block diagram showing the sensing elements of thesystem.

FIG. 6 is a flow chart diagram outlining an exemplary method of volumemeasurement.

FIG. 7 is a flow chart outlining an exemplary method of calculating flowrate based on pressure decay.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, wherein like numerals reference numerals areused to indicate like or analogous components throughout the severalviews, FIG. 1 depicts an exemplary volume and flow measurement system inaccordance with an exemplary embodiment of the present invention. Thesystem includes a pressure frame 10 that is of known total volume andcontains within it an air bladder 20, and a flexible bag 30 thatcontains within it a liquid to be infused 40.

Referring now to FIG. 2, the air bladder 20 is connected to an air pump50 via a bladder connection line 608, a bladder valve 106, and a bladdervalve line 606. The air bladder 20 may be vented to atmosphere via abladder vent valve 108.

A calibration tank 60 of known volume is connected to the air pump 50via a tank connection line 604, a tank valve 102, and a tank valve line602. The tank 60 may be vented to atmosphere via a tank vent valve 104.

The liquid 40 is fluidically coupled to an output 500 via a liquid drainline 610, going through a fluid flow resistor 400 and through an outputline 612. The liquid 40 may be, for example, a medication fluid,intravenous solution, or the like, and the output 500 may be, forexample, a patient or subject in need thereof.

The tank 60 is connected to a tank pressure sensor 204 and an optionaltank temperature sensor 304. The bladder 20 is connected to a bladderpressure sensor 202 and an optional bladder temperature sensor 302.

Referring now to FIG. 4, an electronic module includes a processing unit700 such as a microprocessor, microcontroller, controller, embeddedcontroller, or the like, and is preferably a low cost, high performanceprocessor designed for consumer applications such as MP3 players, cellphones, and so forth. More preferably, the processor 700 is a moderndigital signal processor (DSP) chip that offers low cost and highperformance. Such processors are advantageous in that they support theuse of a 4th generation programming environment that may substantiallyreduce software development cost. It also provides an ideal environmentfor verification and validation of design. It will be recognized thatthe control logic of the present development may be implemented inhardware, software, firmware, or any combination thereof, and that anydedicated or programmable processing unit may be employed. Alternatelythe processing unit 700 may be a finite state machine, e.g., which maybe realized by a programmable logic device (PLD), field programmablegate array (FPGA), field programmable object arrays (FPOAs), or thelike. Well-known internal components for processor 700, such as powersupplies, analog-to-digital converters, clock circuitry, etc, are notshown in FIG. 3 for simplicity, and would be understood by personsskilled in the art. Advantageously, the processing module may employ acommercially available embedded controller, such as the BLACKFIN® familyof microprocessors available from Analog Devices, Inc., of Norwood,Mass.

With continued reference to FIG. 4, the processing unit 700 controls theair pump 50 via a pump control line 750. The processor 700 controls thetank vent valve 104 via a tank vent valve control line 704. Theprocessor 700 controls the tank valve 102 via a tank valve control line702. The processor 700 controls the bladder vent valve 108 via a bladdervent valve control line 708. The processor 700 controls the bladdervalve 106 via a bladder valve control line 706.

With reference now to FIG. 5, the processor 700 can measure pressure andtemperature from the bladder 20 and tank 60. The processor 700 reads thepressure in the tank 60 via a tank pressure sensor 204, which is coupledto the via tank pressure line 724. The processor 700 reads the pressurein the bladder 20 via a bladder pressure sensor 202, which is coupled tothe processor 700 via a tank pressure line 722. The processor 700 readstemperature of the gas in the tank 60 via a tank temperature sensor 304,which is coupled to the processor 700 via a tank temperature line 714.The processor 700 reads the temperature of the gas in the bladder 20 viaa bladder temperature sensor 302, which is coupled to the processor 700via a bladder temperature line 712.

Volume Measurement

Ultimately, the objective of volume measurement is to know the quantityof liquid 40 remaining in an infusion and how that quantity changes overtime.

The pressure frame 10 defines a rigid container of known volume,V_(frame). This volume is known by design and is easily verified bydisplacement methods. Within the pressure frame 10, there is the airbladder 20, which has a nominal capacity greater than the volumeV_(frame). When expanded, the bladder must conform to the geometry ofthe rigid container and its contents. The volume of liquid 40 to beinfused, V_(tbi), is equal to V_(frame), less the fixed and known volumeof the bladder 20 itself, V_(blad), less any incompressible materials ofthe bag 30, V_(bag), and less the volume of gas in bladder 20, V_(gas).Once the value V_(gas) is computed, then it is trivial to computeV_(tbi).

V _(tbi) =V _(frame) −V _(blad) −V _(bag) −V _(gas)

With the following method, at any given point in time, the volume of aircontained in the bladder, V_(gas), can be measured and V_(tbi) can besubsequently computed.

For purposes of economy and flexibility, the pump 50 may be an impreciseair pump, such as that of a rolling diaphragm variety, although othertypes of pumps are also contemplated. The output of such a pump may varysignificantly with changes in back pressure, temperature, age of thedevice, power supply variation, etc. One advantage of the device andmethod disclosed herein is that they allow an imprecise pump to be usedin a precision application, by calibrating the pump in situ.

FIG. 6 shows the steps leading to computation of V_(tbi). Shown as step802, the first step is to find an optimum amount of air mass, N_(pump),to add to the bladder to effect a significant pressure change, forexample, on the order of about 10%. If the amount of air mass added tothe bladder is too small, then the pressure change will not bemeasurable with accuracy. If the amount of the air mass is too great,then pressure in the bladder will increase more than necessary andenergy will be wasted.

The initial pressure in the bladder 20, P_(bladder1), is measured usingthe bladder pressure sensor 202. The tank valve 102 is set to a closedstate via the tank control valve line 702 from the processor 700. Thebladder valve 106 is set to an open state via the tank control valveline 706 from the processor 700. The pump 50 is activated by theprocessor 700 via the pump control line 750 for a period of time,S_(test), nominally, for example, about 250 milliseconds. A newmeasurement of the pressure in the bladder 20 is made, P_(bladder2).Based on the percent of pressure change from this pumping action, a newpump activation time, S_(pump), will be computed. This calculation needsno precision; it is only intended to find an amount of pumping thatprovides a significant change in pressure, P_(deltatarget), in bladder20, for example, on the order of about 10%.

$S_{pump} = {S_{test}*\frac{P_{deltatarget}}{( {P_{{bladder}\; 2} - P_{{bladder}\; 1}} )/P_{{bladder}\; 1}}}$

In step 804, the pump 50 or the tank vent valve 104 are activated toincrease or decrease, respectively, the pressure, P, in the tank 60, sothat it approximately equals the pressure, P_(bladder), in bladder 20.The combination of valve and pump settings required for such adjustmentsare shown in the table below:

Bladder Bladder Tank Pump Valve Vent Valve Tank Vent 10 106 Valve 108102 Valve 104 Increase P_(bladder) ON OPEN CLOSED CLOSED CLOSED DecreaseP_(bladder) OFF CLOSED OPEN CLOSED CLOSED Increase P_(tank) ON CLOSEDCLOSED OPEN CLOSED Decrease P_(tank) OFF CLOSED CLOSED CLOSED OPEN

Adjustments made in step 804 can be made iteratively until P_(tank) isroughly equal to P_(bladder), for example, within about 5% of therelative pressure measured in P_(bladder). This does not need to be aprecise process. Following the adjustment, the pressure in tank 60,P_(tank2), is recorded.

In step 806, the system is configured to increase the pressure in tank60, as shown in the above table. The pump 50 is activated for a timeperiod equal to S_(pump) After a delay of approximately five seconds,the pressure in the tank 60 is measured, P_(tank3). This delay is toreduce the effect of an adiabatic response from the increase in pressurein the tank 60.

In step 808, the system is configured to increase the pressure inbladder 20, as shown in the above table. The pump 50 is activated for aperiod equal to S_(pump). After a delay of approximately five seconds,the pressure in the bladder 20 is measured, P_(bladder3). This delay isto reduce the effect of an adiabatic response from the increase inpressure in the bladder 20.

Because the initial pressures in the bladder 20 and the tank 60 wereapproximately equal, the quantity of air mass injected into tank 60 instep 806 and into bladder 20 in step 808 will be roughly equal, eventhough the pump 50 need not be a precise metering device.

We take advantage of several simplifications. First, the ambienttemperature for sequential steps 806 and 808 is unchanged. Second, theatmospheric pressure during sequential steps 806 and 808 is unchanged.These conditions simplify the ideal gas law formula and allow the use ofgauge pressure measurements, rather than absolute pressure.

In step 810, the volume of gas in the bladder 20, V_(gas), can becalculated with a reduced form of PV=nRT:

$V_{gas} = \frac{V_{tank}*( {P_{{tank}\; 3} - P_{{tank}\; 2}} )}{( {P_{{bladder}\; 3} - P_{{bladder}\; 2}} )}$

As examples of this calculation, if the pressure change were the same inthe bladder 20 and the tank 60, then V_(gas) would be equal to V_(tank).If the pressure change in the bladder 20 were 20% as large as that inthe tank 60, then V_(gas) would be 5 times greater than V_(tank).

Step 812 derives the value for V_(tbi) from V_(gas), using known valuesfor V_(frame) Vblad, and V_(bag) and using the calculated value ofV_(gas), from step 810.

V _(tbi) =V _(frame) −V _(blad) −V _(bag) −V _(gas)

The valves 102, 106, 104, and 108 can be configured in many ways,including multiple function valves and or manifolds that toggle betweendistinct states. The depiction herein is made for functional simplicityand ease of exposition, not necessarily economy or energy efficiency.

Flow Rate Calculation

Once the fluid volume has been computed, multiple measurements made overtime will yield knowledge of fluid flow rate, which is, by definition,fluid volume changing over time. Repeated measurements of volume overtime provided more and more resolution of average flow rate. The averageflow rate and the volume of liquid 40 remaining to be infused can beused to estimate the time at which the fluid volume will be delivered.If the infusion is to be completed within some specified period of time,any error between the specified time and the estimated time can becalculated and the flow rate can be adjusted accordingly.

There are situations where the short-term flow rate is of interest.Rather than make repeated volume measurements over a short period oftime, there is an alternative approach. Once the gas volume in bladder20 is known, then the observation of pressure decay in the bladder canbe converted directly to a flow rate. It is important to know that themeasurement of pressure decay, by itself, is not adequate to computeflow rate. For example, if the pressure were decaying at a rate of 10%per hour, this information cannot be converted into flow rate, unlessthe starting gas volume is known. As an example, if V_(gas) has beenmeasured to be 500 ml and the absolute pressure is decaying at a rate of5% per hour, then the flow rate is 5% of 500 ml per hour or 25 ml perhour. The knowledge of the initial volume is critical to compute fluidflow rate.

The measurement of pressure decay is a simple procedure of observing thetime the absolute pressure of P_(bladder) to drop by a small, butsignificant, amount, preferably for example about 2%. Because theprocessor 700 is capable of measuring times from microseconds to years,this measurement carries a very wide dynamic range. By observing a 2%drop, the change in pressure is well above the noise floor of thepressure measurement system.

A flow chart outlining an exemplary process 900 for calculating flowrate by monitoring the rate of pressure decay in the bladder 20 is shownin FIG. 7. At step 904, the volume of gas in the bladder 20 iscalculated as detailed above. At step 908, the pressure in the bladder20, P_(bladder1) is measured using the sensor 202 at time T1, which isrecorded in step 912. The pressure in the bladder 20 is measured againat step 916 and the time T2 is recorded at step 920. The change inpressure, ΔP, between the time T1 and the time T2 is calculated in step924 as P_(bladder1)−P_(bladder2) and the change in time, ΔT iscalculated as T2-T1 at step 928. At step 932, it is determined whetherΔP is greater than some predetermined or prespecified threshold value,e.g., about 2% with respect to P_(bladder1) If ΔP has not reached thethreshold value at step 932, the process returns to step 916 andcontinues as described above. If ΔP has reached the threshold value atstep 932, the rate of pressure decay is calculated as ΔP/ΔT at step 936.The flow rate is then calculated as ΔP/ΔT×V_(gas)−P_(bladder1) at step940.

The invention has been described with reference to the preferredembodiments. Modifications and alterations will occur to others upon areading and understanding of the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. A method of measuring a volume of liquid in a flexible container,comprising: placing the flexible container within a first rigidcontainer of known volume, the first rigid container containing aninflatable bladder; pressurizing the inflatable bladder with a gas;pressurizing a second rigid container of known volume with a gas, thepressure of the gas in the bladder being approximately equal to thepressure of the gas in the second rigid container; delivering a firstmolar quantity of gas to the bladder to cause a measurable increase inpressure in the bladder; delivering a second molar quantity of gas tothe second rigid container to cause a measurable increase in pressure inthe second rigid container, the first molar quantity of gas beingapproximately equal to the second molar quantity of gas; measuring theincrease in pressure in the bladder; measuring the increase in pressurein the second rigid container; calculating the volume of gas in thebladder using the known volume of the second rigid container, themeasured increase in pressure in the bladder, and the measured increasein pressure in the second rigid container; and calculating the volume ofliquid in the flexible container by subtracting the calculated volume ofgas in the bladder from the known volume of the first rigid container.2. The method of claim 1, further comprising: subtracting the knownvolume of incompressible materials within the first rigid container fromthe known volume of the first rigid container.
 3. The method of eitherone of claims 1 and 2, wherein said gas is air.
 4. The method of any oneof claims 1-3, wherein said gas is delivered to said first and secondrigid containers with a pump.
 5. The method of claim 5, wherein saidpump is not a precise metering device.
 6. The method of any one ofclaims 1-5, further comprising: prior to delivering said first andsecond molar quantities of gas, adjusting the pressure in one or both ofsaid bladder and said second rigid container.
 7. The method of any oneof claims 1-6, further comprising: monitoring the pressure in saidbladder; and calculating a flow rate of liquid exiting the flexiblecontainer.
 8. A method for calculating a flow rate of liquid exiting aflexible container contained within a rigid container of known volume,the rigid container containing an inflatable bladder, the inflatablebladder pressurized with a gas to urge the liquid out of the flexiblecontainer, said method comprising: calculating the volume of gas in theinflatable bladder; determining the initial pressure in the inflatablebladder; monitoring the pressure decay in the inflatable bladder overtime until the change in pressure reaches some preselected thresholdvalue; calculating the rate of pressure decay; and calculating the flowrate using the rate of pressure decay, the calculated volume of gas inthe inflatable bladder, and the initial pressure in the inflatablebladder.
 9. A fluid delivery system, comprising: a pressure frame (10)of known total volume; an inflatable bladder (20) within said pressureframe; said pressure frame adapted to receive a flexible bag (30)containing a liquid to be infused (40); a calibration tank (60) of knownvolume; a pump (50) fluidically coupled to said bladder and saidcalibration tank for selectively delivering a gas to said bladder andsaid calibration tank; a first pressure sensor (202) coupled to saidbladder for sensing the pressure of a gas in said bladder; a secondpressure sensor (204) coupled to said calibration tank for sensing thepressure of a gas in said calibration tank; a processing unit (700)coupled to said first and second pressure sensors and said first andsecond temperature sensors for storing pressure and temperatureinformation from said first and second pressure sensors and said firstand second temperature sensors; said processing unit coupled to saidpump for controlling operation of said pump; and said processing unitfurther including means for calculating one or both of: a volume ofliquid in the flexible container; and a flow rate of fluid exiting theflexible container.
 10. The fluid delivery system of claim 9, furthercomprising: a first vent valve (108) fluidically coupled to said bladderfor selectively venting gas within said bladder; and a second vent valve(104) fluidically coupled to said calibration tank for selectivelyventing within said calibration tank.
 11. The fluid delivery system ofeither one of claims 9 and 10, further comprising: a first inlet valve(106) fluidically coupled to said bladder and said pump; and a secondinlet valve (102) fluidically coupled to said calibration tank and saidpump.
 12. The fluid delivery system of any one of claims 9-11, furthercomprising: a first temperature sensor (302) coupled to said bladder forsensing the temperature of the gas in said bladder; and a secondtemperature sensor (304) coupled to said calibration tank for sensingthe temperature of a gas in said calibration tank.
 13. The fluiddelivery system of any one of claims 9-12, wherein the gas is air. 14.The fluid delivery system of any one of claims 9-13, wherein said pumpis not a precise metering device.